Highly bright mechanoluminescence material and process for producing the same

ABSTRACT

A novel highly bright mechanoluminescence material free from decay of luminescence brightness even if repeated stress is applied, comprising a composite semiconductor crystal of the general formula xM 1 A 1 .(1-x)M 2 A 2  (wherein each of M 1  and M 2  independently represents an atom selected from among Zn, Mn, Cd, Cu, Eu, Fe, Co, Ni, Mg and Ca, and each of A 1  and A 2  is an atom independently selected from among chalcogens, provided that M 1 A 1  is different from M 2 A 2 ; and x is a positive number less than 1); and a process for producing the same.

TECHNOLOGICAL FIELD

The present invention relates to a novel mechanoluminescence materialor, namely, a material emitting light by conversion of a mechanicalenergy to a light energy as well as to a method for the preparationthereof.

BACKGROUND TECHNOLOGY

There are known heretofore, as a light-emitter which emits light bystimulation from outside, phosphors for fluorescent lamps which aresubject to excitation by ultraviolet light, phosphors for plasmadisplays, phosphors for high-speed electron excitation which are subjectto excitation with electron beams, phosphors for fluorescencecharacter-indicating tubes, phosphors for radiations excitable by aradiation such as X-rays and the like, fluorescence-regeneratingphosphors excitable with heat or infrared light such as those for solidscintillators and the like, accelerated phosphors, infrared-visibleconversion phosphors and others.

As for a material capable of emitting light by an external mechanicalforce, on the other hand, the inventors previously made proposals, inconducting preparation of a high-brightness stress light-emittingmaterial formed from an aluminate having a non-stoichiometriccomposition and having lattice defects emitting light when carriersexcited by mechanical energy return to the ground state or in conductingpreparation of a high-brightness light-emitting material with analuminate as the matrix substance containing, in the above matrixsubstance, metal ions selected from rare earth metal ions or transitionmetal ions as the center ions of the center of luminescence (officialpublication of Japanese Patent Kokai No. 2001-49251), for a method forthe preparation of a high-brightness light-emitting material (officialpublication of Japanese Patent Kokai No. 2002-220587) characterized bycomprising: mixing an aluminum alcoholate and water-soluble compounds ofthe ingredient metals other than aluminum in an aqueous medium followedby conversion into alkalinity so as to form a colloid, then subjectingthe same to high-speed drying with addition of a dispersion-stabilizingagent to form a dried material having the dispersion-stabilizing agentdeposited on the surface of the colloidal particles followed bycalcination of this dried material in an oxidizing atmosphere at 500 to900° C. and pulverizing the thus calcined material into a powder whichis, before or after molding, fired in a reducing atmosphere at 1000 to1700° C.

However, the mechanoluminescence materials known heretofore haveunavoidably limited application fields in respect of their insufficientluminescence brightness and in respect of attenuation of theirluminescence brightness as a stress is repeatedly applied thereto.

DISCLOSURE OF THE INVENTION

The present invention has been completed under these circumstances withan object to provide, by overcoming the defects possessed by the priorart mechanoluminescence materials, a novel mechanoluminescence materialhaving high brightness and free from attenuation of the luminescencebrightness even by repeated application of a stress.

The inventors have continued extensive investigations in order toimprove the defects possessed by the prior art mechanoluminescencematerials and, as a result, to develop a novel mechanoluminescencematerial exhibiting a greatly improved efficiency for the conversion ofa mechanical energy to an energy of light and, as a result, have arrivedat a discovery that a material having a constitution bycomposite-formation of a specified semiconductor and, by modifying thepreparation method, having adequately controlled crystalline grain sizeand defects or strain of the crystalline lattice can be a stable andhigh-brightness mechanoluminescence material leading to completion ofthe present invention on the base of this discovery.

Namely, the present invention provides a high-brightnessmechanoluminescence material consisting of a composite semiconductorcrystal represented by the general formulaxM¹A¹.(1-x)M²A²  (I)(M¹ and M² in the formula are, each independently from the other, anatom selected from among Zn, Mn, Cd, Cu, Eu, Fe, Co, Ni, Mg and Ca, A¹and A² are, each independently, an atom selected from chalcogens withthe proviso that M¹A¹ and M²A² differ each from the other and x is apositive number smaller than 1),as well as a method for the preparation of a high-brightnessmechanoluminescence material comprising the steps that the sourcematerials of the constituent elements are blended in a specifiedproportion, the thus obtained mixture is subjected to heating in vacuumat a temperature lower than the sublimation point of the product to forma composition corresponding to the aforementioned general formula (I),the said composition is subjected to sublimation at a temperature equalto or higher than the sublimation point of the said composition and thegenerated sublimate is condensed at a temperature lower than thesublimation point to effect crystallization.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a plan view showing a model of the wurtzite-type structure.

FIG. 2 is a perspective view showing a model of the zincblende-typestructure.

FIG. 3 is an illustration showing the structure of the rubbing-testingmachine used in Example 1.

FIG. 4 is a graph showing the changes of the luminescence intensity inthe lapse of time in Example 1.

FIG. 5 is a graph showing the relationship between loading weight andluminescence intensity in Example 1.

BEST MODE FOR CARRYING OUT INVENTION

The present invention is constituted of a composite body of asemiconductor represented by the general formula M¹A¹ (II) and asemiconductor represented by the general formula M²A² (III). The M¹ andM² in these general formulas are selected from among Zn, Mn, Cd, Cu, Eu,Fe, Co, Ni, Mg and Ca and they can be a single kind or a combination oftwo kinds or more with respect to each of the M¹ and M². Thoughdepending on the cases, it is possible to replace a part of these metalswith other metals known for replacement in conventional semiconductorssuch as, for example, Al or Ga for Zn, Hg for Cd and Sr and the like forCa.

In the next place, the A¹ and A² which are each a non-metal usable toform a semiconductor by being bonded with these metals includechalcogens or, namely, the oxygen group elements. The chalcogens includeoxygen, sulfur, selenium, tellurium and polonium of which polonium isless preferable due to its high metallic nature. While it is usual thatthese chalcogens are used singly relative to each of the A¹ and A², theycan be a combination of two kinds or more, if so desired. It is furtheroptional to replace them with other non-metallic elements such as, forexample, silicon, nitrogen, boron, arsenic and the like within a rangenot to cause a decrease in the properties as the semiconductor.

The mechanoluminescence material of the present invention is preferablyconstituted of an oxide, sulfide, selenide or telluride as the principalingredients having a structure With coexistence of the wurtzite-typestructure and zincblende-type structure. The wurtzite-type structurehere implied is a crystalline structure in which the positive element M¹(marked with ●) and the negative element A¹ (marked with ◯) arranged asshown in FIG. 1 which is formed by M¹A¹ ₄ tetragons jointly possessingeach of the corners. And, the zincblende-type here implied is, as isillustrated in FIG. 2, a structure in which unit layers of awurtzite-type structure formed from M¹ (● marks) and A¹ (◯ marks) arelaid one on the other in such a fashion that the M¹s (● marks) arearranged in the cubic closest packing structure.

In the mechanoluminescence materials of the present invention, it ispreferable in respect of the particularly high stress luminescenceintensity that the M¹ is Mn and the negative elements of M¹A¹ and M²A²or, namely, A¹ and A², are each the same chalcogen as the other in thecomposition as exemplified by the compositions of, for example,Mn_(x)Zn_(1-x)S, Mn_(x)Zn_(1-x)Te, Mn_(x)Cd_(1-x)S,Mn_(x)Zn_(1-x-y)Cu_(y)S (where 0<x<0.5 and 0<y<1-x). Furthermore, amaterial exhibiting a strong stress luminescence like a laser beam canbe obtained when the material is constituted from microcrystals havingno or minimum strains.

In the following, a description is given of a method for the preparationof the mechanoluminescence material of the present invention. The sourcematerials for forming the semiconductor represented by the above-givengeneral formula (II) and the source materials for the semiconductorrepresented by the above-given general formula (III) are blended in aspecified proportion corresponding to x in the range of, for example,from 0.01 to 99.99 or, preferably, from 0.1 to 99.9 and the same takento fill a quartz tube is heated under vacuum to cause, by utilizing thetemperature gradient, sublimation of the starting materials at thehigher temperature end and re-crystallization at the lower temperatureend. It is preferable in this case to use a small amount of a chalcogenor halogen as a circulation gas to promote the sublimation. Thiscirculation gas is used in the range from 0.01 to 10 mg per unit volume1 cm³ of the quartz tube.

In this way, a desired semiconductor is synthesized from the startingmaterials filling the quartz tube at a temperature lower than thesublimation point followed by sublimation of the same at a temperatureequal to or higher than the sublimation point to cause condensation byleading the same to a low-temperature zone so that a high-brightnessmechanoluminescence material is obtained. In this case, the quartz tubeis evacuated to vacuum and, after flushing with argon or hydrogen, theinside thereof is brought to high vacuum of 10⁴ Pa or below or,preferably, 10⁻² Pa or below so as to accomplish improvement of thelight-emitting intensity.

In the mechanoluminescence material of the present invention, the stressluminescence intensity depends on both of the crystal grain diameter andthe lattice strain. Namely, the luminescence intensity is decreased asthe crystal grain diameter is increased and a high luminescenceintensity is exhibited as the crystal grain diameter is decreased, forexample, to nano-size crystal grains. It is usually preferable that thecrystal grain diameter does not exceed 35 nm or, in particular, does notexceed 20 nm. The crystal grain diameter can be determined by X-raydiffractometry.

Further, the luminescence intensity is greatly decreased in theexistence of strain in the grains so that fine grains having littlestrains should be prepared to obtain a high luminescence intensity.Simultaneous analyses can be undertaken by the X-ray diffractometry forthe crystal grain size and strain in the crystals. Namely, the crystalgrain size and the strain in the lattice can be simultaneously obtainedby bringing the pseudo-Voigt equation into profile fitting to adiffractometric diagram of a broad range to independently determine thehalf-value widths in the Gaussian distribution and Lorentz distribution(see “Material analyses by powder X-ray diffractometry” by TakamitsuYAMANAKA, published by Kodan-sha, Jun. 1, 1993, page 95).

The luminescence intensity of the mechanoluminescence material of thepresent invention is subject to changes depending on the value of themechanical energy to be the excitation source or, namely, the mechanicalinteraction. While the luminescence intensity of a mechanoluminescencematerial is generally increased as the mechanical interaction force isincreased, a smallest energy or, namely, a threshold value is found inthe mechanical interaction force to cause light emission. While thisthreshold value is subject to changes depending on the composition ofthe material with wide variations ranging from those emitting light evenwith a small energy of less than 1N to those capable of emitting lightonly by application of a large energy to approach destruction of thematerial, the mechanoluminescence material of the present invention canemit light by application of only an extremely small outer force.

While the mechanoluminescence material of the present invention can bein a powder form as such, it can be shaped in the form of a block or acoating film and further it can be worked into a laminated body or acomposite with a plastic.

With regard to the mechanoluminescence material of the presentinvention, it is possible to obtain any ones as desired having a graindiameter ranging from the nano size to the mm size by controlling thefiring conditions in the preparation thereof. And, the ultrafine grainshaving a submicron or smaller grain diameter obtained in this way canemit intense light like laser beams by receiving a stimulation of amechanical outer force and this luminescence is never subject toattenuation with stability even by repeated application of mechanicalouter forces.

In the following, the present invention is described in more details byway of examples although the present invention is never limited by theseexamples.

EXAMPLE 1

A quartz tube was filled with a blend of ZnCuS and MnS in a proportionof 9.9:0.1 by the molar ratio followed by pressure reduction of insideof the quartz tube down to 10⁻² Pa and heating and firing at 900° C. for24 hours. By using an electric furnace suitable for control of thetemperature distribution gradient, thereafter, the above formed ZnCuMnSwas gathered to one end portion of the quartz tube and the temperatureof the portion was increased to 1100° C. while the other end portion waskept at 900° C. to continue the heating for 7 days so that the ZnCuMnSwas crystallized by sublimation at the higher-temperature side andcondensation at the lower-temperature side.

The grain diameter of the fine crystals obtained in this way wasdetermined by the X-ray diffractometry and shown in Table 1.

In the next place, the mechanoluminescence material obtained in thismanner was subjected to light emission by repeating excitation underapplication of a load of 0.2N by using a rubbing testing machine havinga structure illustrated in FIG. 3 (60 rpm revolution, a transparentresin-made rubbing rod having an end point of 1 mm diameter used). FIG.4 is a graph showing the changes with time in the luminescence causedhere. As is understood from this figure, this luminescence is neversubject to attenuation even by repeated application of the load.

The relative value of the stress luminescence intensity here againstSrAl₂O₄:Eu is shown in Table 1.

In the next place, the stress dependency of the luminescence intensitywas obtained by determining the changes in the luminescence intensityunder successive increase of the weight for loading. The results areshown in FIG. 5 as a graph. When this graph is utilized, it is possibleto obtain the value of the mechanical interaction force applied bydetermining the luminescence intensity.

EXAMPLES 2 TO 10

In the same manner as in Example 1, 9 kinds of mechanoluminescencematerials having different compositions and different crystal graindiameters as shown in Table 1 were prepared. The grain diameters of thecrystal grains and the relative luminescence intensities against theluminescence intensity of SrAl₂O₄:Eu taken as 100 as determined forthese materials are shown in Table 1.

TABLE 1 Crystal Relative grain luminescence diameter Sample Compositionintensity (nm) Control SrAl₂O₄:Eu 100 50 Example 1 0.99ZnCuS · 0.01MnS3124000 10 Example 2 0.9ZnTe · 0.1MnTe 1999000 12 Example 3 0.9ZnCuS ·0.1MnS 51300 32 Example 4 0.9ZnTe · 0.1CdSe 4600 25 Example 5 0.9CdS ·0.1MnS 298 30 Example 6 0.9CdSe · 0.1MnTe 980 50 Example 7 0.9ZnS ·0.1MnS 869000 20 Example 8 0.1ZnS · 0.9MnS 5800 500 Example 9 0.9ZnSe ·0.1CdTe 6700 60 Example 10  0.8ZnO · 0.2MnCuS 1880 15

As is understood from this table, a high luminescence intensity isexhibited with those in which, in particular, M¹ is Zn or Zn withpartial replacement with Cu, M² is Mn and A¹ and A² are each the sameelement as the other such as, for example, S or Te. Further, a strongluminescence intensity is exhibited as a trend when the crystal graindiameter does not exceed 20 nm.

INDUSTRIAL UTILIZABILITY

According to the present invention, a novel mechanoluminescence materialis provided which emits strong luminescence by a mechanical outer forcesuch as a rubbing force, shearing force, impact force, pressure,tension, torsion and others and, by using the same, a mechanical energycan be directly converted into the energy of light to give a possibilityof utilization for sensors, displays, amusement instruments, examinationof stress distributions and others.

1. A high-brightness mechanoluminescence material consisting of acomposite semiconductor crystal represented by the formula:xM¹A¹.(1-x)M²A² wherein M¹ is Mn or Eu, M² is Zn, Mn, Cd, Cu, Eu, Fe,Co, Ni, Mg or Ca, each of A¹ and A² is the same chalcogen with theproviso that M¹A¹ and M²A² differ each from the other, and x is apositive number smaller than 1 and wherein the composite semiconductorcrystal has a mixed structure of the wurtzite-type structure and thezincblende-type structure.
 2. A method for the preparation of thehigh-brightness mechanoluminescence material consisting of a compositesemiconductor crystal represented by the formula:xM¹A¹.(1-x)M²A² wherein each of M¹ and M² is, independently from theother, an element selected from Zn, Mn, Cd, Cu, Eu, Fe, Co, Ni, Mg andCa, each of A¹ and A² is an atom selected independently from chalcogens,with the proviso that M¹A¹ and M²A² differ each from the other, and x isa positive number smaller than 1 and wherein the composite semiconductorcrystal has a mixed structure of the wurtzite-type structure and thezincblende-type structure, which comprises the steps of mixing sourcematerials of the constituent ingredients; heating the thus obtainedmixture in vacuum at a temperature lower than the sublimation point ofthe product to produce a composition represented by the formulaxM¹A¹.(1-x)M²A² wherein each of M¹ and M² is, independently from theother, an element selected from Zn, Mn, Cd, Cu, Eu, Fe, Co, Ni, Mg andCa, each of A¹ and A² is an atom selected independently from chalcogensand x is a positive number smaller than 1, with the proviso that M¹A¹and M²A² differ each from the other; causing sublimation of thecomposition at a temperature equal to or higher than the sublimationpoint of the composition; and crystallizing the thus generated sublimateby condensation at a temperature lower than the sublimation pointthereof.